Process for coating substrates with a silicium-containing layer

ABSTRACT

A process for coating substrates with a silicon-containing protective layer by chemical vapor deposition with a silicon-containing compound of the structural formula (1):                    
     in which 
     R 1  is an alkyl group having 1 to 4 carbon atoms, and 
     R 2  is hydrogen or an alkyl group having 1 to 4 carbon atoms.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for the coating of substrates with asilicon-containing protective layer by chemical vapour deposition (CVD)coating.

2. Description of the Prior Art

In order to protect high-temperature components of carbon from oxidationit is known to provide the component with a quartz glass (SiO₂) layer byCVD using silanes. The amorphous quartz glass layer passes into thecrystalline state (cristobalite) at a temperature above approximately1,100° C. This so-called transition of quartz leads to cracks in thecoating which lead to rapid oxidation of the carbon component, inparticular following cooling and re-heating of the component, that is tosay under conditions of thermal shock stress.

Although cracking can be checked by applying further layers, for exampleof silicon carbide, such a sequence of different layers is associatedwith a corresponding number of process steps and is hence costly andtime-consuming. Moreover, a quartz glass layer applied to a metalsubstrate by CVD leads to spalling under conditions of mechanical andthermal shock stress.

The object of the invention is to provide substrates of different typesin simple manner with a firmly adhering protective layer which alsowithstands high thermal shock stresses.

This is achieved according to the invention as decribed below.

SUMMARY OF THE INVENTION

In accordance with the invention, a compound of the following structure(1):

in which

R¹ is an alkyl group having 1 to 4 carbon atoms, and

R² is hydrogen or an alkyl group having 1 to 4 carbon atoms is used asthe starting compound for the CVD process for the coating of thesubstrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

R¹ is preferably a methyl radical and R² hydrogen. That is to say,tri(dimethylaminosilyl)-amino-di(dimethylamino) borane which has thefollowing structural formula (2):

is preferably used.

A thermal CVD process, in particular an LPCVD (low-pressure CVD)process, is preferably employed as the CVD process. However, other CVDprocesses, in particular plasma CVD, may also be used according to theinvention in place of the thermal CVD process.

The apparatus for the thermal CVD process preferably has apressure-tight supply vessel which contains the liquid starting compoundaccording to the formula (1) or (2) and is under pressure from an inertgas, for example argon. The liquid starting compound is supplied by wayof a flow measuring device to a mixing means into which an inert gas,for example nitrogen, flows at the same time by way of a correspondinggas flow measuring device, as a result of which there is formed from theliquid starting compound in the mixing means an aerosol which isvaporised without residue in a heated vaporiser. The vapour is suppliedto one end of a coating oven of preferably tubular construction, intowhich the substrate to be coated or a plurality of substrates arearranged above one another and/or in sequential manner. A vacuum pump isconnected to the other end of the tubular oven.

When tri(dimethylaminosilyl)-amino-di(dimethylamino) borane is used asthe starting compound, the vaporiser temperature is, for example, from40° C. to 150° C., preferably 75° C. to 100° C. The negative pressure inthe coating oven can be from 10⁻¹ to 10⁻⁵; it is normally around 10⁻² to10⁻³ bar.

The deposition conditions can be maintained in precise manner using theCVD apparatus which is described, thus enabling layers having highlyreproducible properties to be obtained.

In the coating oven the substrate is heated to a temperature of from400° C. to 1800° C., in particular 650° C. to 1400° C. This enables alayer to be produced which remains amorphous at up to 1900° C.

The layer produced by the process according to the invention containsthe following elements (wherein this term also includes the elementsbonded together): silicon, nitrogen, boron and carbon. The elements Si,N and B may be present, as in the starting compound, in a molar ratio of1:1:1. The layer may also contain, in addition to the elements Si, N, Band C, organic residues formed from the starting compound whichinfluence the properties of the layer. If these organic residues are tobe removed from the layer, the substrate is heated to a correspondinglyhigh temperature. The CVD deposition in the oven may, however, also takeplace with the substrate at a temperature which is rather lower, and athermal post-treatment be carried out at from 600° C. to 1800° C. in anoven, in order to expel the organic residues.

The Si—N—B—C layer thus formed is in particular suitable for protectingmetal parts. The metal parts may be of, for example, steel or a titaniumalloy.

The protective layer applied to the metal part by the process accordingto the invention is distinguished by high adhesion. This is inparticular extremely successful if the metal part is coated in theunpolished state, for instance has a peak-to-valley height of more than5 #m.

The layer which is applied to the metal part by the process according tothe invention has high wear resistance and lubricating properties inaddition to the high adhesion. The former properties can be influencedby the content of organic residues deriving from the alkyl groups of thestarting substrate.

Owing to the excellent tribological properties of the ceramic protectivelayer created according to the invention, the process according to theinvention may be used, for example, for coating metal parts inautomotive construction.

In addition to the coating of metal substrates, the process according tothe invention is above all also eminently suitable for the coating ofcarbon substrates or of composite substrates prepared from carbon andsilicon, which are exposed to high temperature, in particular to highthermal shock stress, for example in space travel, for example for jetengine nozzles. It is furthermore suitable for the coating of ceramicsubstrates.

If the carbon, silicised carbon or ceramic components coated with theSi—N—B—C protective coating by the process according to the inventionare heated to temperatures of, for example, from 900° C. to 1800° C., inparticular 1200° C. to 1600° C., in an oxygen-containing atmosphere suchas, for example, air, the Si at the surface of the protective coatingoxidises to SiO_(x), that is to say SiO and SiO₂.

The latter oxidation may take place as a result of post-treating thecoated substrate in an oven, or during use of the component at hightemperatures in the atmosphere.

The SiO_(x) which is formed at the surface of the component has arelatively low melting point owing to the boron. Consequently, theprotective layer in the surface region melts even when the temperatureis relatively low, and the melt seals any cracks which may arise in theunderlying region of the protective layer, as a result of which oxygenpenetration to the substrate of carbon or of carbon/silicon isprevented. That is to say, the SiO_(x) content in the surface of theprotective layer is effective at high temperatures, while the Si—N—B—Cphase of the remaining protective layer, that is to say the base layer,protects the component from oxidation at low temperatures. The same istrue of the nitrogen-containing substrates.

As a result of the process according to the invention, a protectivelayer is created which protects the coated component from oxidation atup to approximately 1900° C. in reliable manner even under conditions ofthermal shock stress in accordance with the processes hitherto.

The Examples which follow serve to explain the process according to theinvention, while not limiting it.

EMBODIMENT EXAMPLES Example 1

Two substrates of reaction-bonded Si₃N₄ were heated to 900° C. in thecoating oven. 2 ml of the starting compoundtri(dimethylaminosilyl)-amino-di(dimethylamino) borane were vaporised at90° C. and guided through the coating oven. The pressure in the oven was5×10⁻⁵ bar. After 90 minutes the starting compound was exhausted, andthe substrates were cooled to room temperature. One of the twosubstrates was coated again under identical conditions. The substrateswere then aged in nitrogen at 1450° C. for 1 hour. The coatings coveredthe substrates evenly. Scanning electron (SEM) and transmission electron(TEM) micrographs showed the intimate bond between the substrate and thefirst coating, and between the first and second coatings. Both appliedlayers were amorphous. Electron-dispersive X-ray (EDX) analyses showedthe layer to contain Si, N, B and C. The total layer thickness, measuredon the twice-coated sample, was 7 to 8 μm.

Example 2

Substrates of reaction-bonded Si₃N₄ were heated to 950° C. in thecoating oven. 4 ml of the starting compoundtri(dimethylaminosilyl)-amino-di(dimethylamino) borane were vaporised at80° C. and guided into the coating oven. The pressure in the oven was2×10⁻⁵ bar. After the starting compound had vaporised the substrateswere cooled to room temperature. The substrates were coated a secondtime under identical conditions. The substrates were then aged innitrogen at 1450° C. for 2 hours. The coatings covered the substratesevenly.

Example 3

A substrate of graphite was heated to 1000° C. in the coating oven. 1 mlof the starting compound tri(dimethylaminosilyl)-amino-di(dimethylamino)borane was vaporised at 100° C. and guided through the coating oven. Thepressure in the oven was 2.7×10⁻³ bar. After the starting compound hadvaporised the substrate was cooled to room temperature. The substratewas then aged in nitrogen at 1450° C. for one hour. The coating coveredthe substrate evenly. Scanning electron microscopic (SEM) examinationshowed an intimate bond between the substrate and the coating.Measurements taken using X-ray micro-analysis showed Si, N, B and C inthe layer. The layer thickness was 2.6 μm.

What is claimed is:
 1. A process for coating a substrate with asilicon-containing protective layer, by chemical vapor deposition usinga starting compound of the structural formula (1):

in which R¹ is an alkyl group having 1 to 4 carbon atoms, and R² ishydrogen or an alkyl group having 1 to 4 carbon atoms.
 2. The processaccording to claim 1, wherein the starting compound istri(dimethylaminosily)-amino-di(dimethylamino)borane.
 3. The processaccording to claim 1, wherein the chemical vapour deposition is carriedout with thermal or plasma means.
 4. The process according to claim 1,wherein the pressure is from 10⁻¹ to 10⁻⁵ bar.
 5. The process accordingto claim 1, wherein the substrate is coated at a temperature of from400° C. to 1800° C.
 6. The process according to claim 1, furthercomprising the step of thermally post-treating the coated substrate at atemperature of from 600° C. to 1800° C.
 7. The process according toclaim 6, wherein the thermal post-treatment step is carried out in anoxygen-containing atmosphere.
 8. The process according to claim 1,wherein the substrate contains a metal or at least one element selectedfrom the group consisting of carbon, silicon and nitrogen.
 9. Theprocess according to claim 8, wherein the substrate is a metalsubstrate.
 10. The process according to claim 9, wherein the metalsubstrate is unpolished.
 11. The process according to claim 1, which iscarried out in a tubular coating oven.
 12. The silicon-containingarticle of manufacture comprising a substrate having adhered thereto asilicon-containing protective layer

prepered from the process according to claim
 1. 13. Thesilicon-containing article of manufacture according to claim 12, whereinthe substrate contains a metal or at least one element selected from thegroup consisting of carbon, silicon and nitrogen.
 14. Thesilicon-containing article of manufacture according to claim 12, whereinthe substrate is coated at a temperature of from 400° C. to 1800° C. 15.The silicon-containing article of manufacture according to claim 12,wherein the coated substrate is thermally post-treated at a temperaatureof from 600° C. to 1800° C.
 16. The silicon-containing article ofmanufacture according to claim 12, wherein the substrate is coated bychemical vapor deposition which is carried out with thermal or plasmameans.
 17. The silicon-containing article of manufacture according toclaim 12, wherein the substrate is coated at a pressure from 10⁻¹ to10⁻⁵ bar.
 18. The silicon-containing article of manufacture according toclaim 12, wherein the starting compound istri(dimethylaminosilyl)-amino-di(dimethylamino) borane.